{	Problem: http://codeforces.ru/problemset/problem/54/A
	Verdict: Accepted
}
uses sysutils;
var i,n,k,c,p,r,j:longint; a:array[0..370] of boolean;
begin
fillchar(a,sizeof(a),0);
read(n,k);
read(c);
for i:=1 to c do begin
  read(j);
  a[j]:=true;
end;
p:=0;
r:=0;
for i:=1 to n do
  if a[i] or (i-p=k) then begin
    inc(r);
    p:=i;
  end;
write(r);
end.